Prior systems have been devised wherein a generator is operated as a motor to develop motive power which is transferred to a prime mover to start same, thus obviating the necessity of a dedicated starter motor or other starting apparatus For example, Cordner U.S. Pat. No. 4,315,442, assigned to the assignee of the instant application, discloses such a system wherein motive power developed by the generator is transferred through a constant speed drive to start the prime mover. A similar system is disclosed in Aleem U.S. Pat. No. 3,786,696. In the former system the power is transferred to the prime mover through a differential of the constant speed drive whereas in the latter motive power developed by the generator is transferred through one of the hydraulic units of the constant speed drive to the prime mover. In both types of systems, inefficiencies are encountered owing to the transfer of power through mechanical or hydraulic components. Also, a failure or malfunction of the constant speed drive can prevent starting of the prime mover by the generator.
Hoffmann et al. U.S. Pat. No. 4,093,869 discloses a method and apparatus for starting a prime mover using a brushless synchronous dynamoelectric machine. The machine includes an exciter having a direct axis field winding which is provided power during operation in a generating mode to in turn provide excitation for a main generator. The exciter also includes a quadrature axis winding which receives AC power during operation in a starting mode so that excitation is available for the main generator even when rotor speed is at zero. During operation in the starting mode, the stator windings of the main generator are provided AC power so that the main generator and the prime mover are accelerated from standstill up to prime mover self-sustaining speed.
A further type of starting/generating system and method is disclosed in Mehl et al. U.S. Pat. No. 4,481,459, assigned to the assignee of the instant application. This system includes a prime mover which is connected to a brushless generator by a torque converter. During operation in the starting mode, the torque converter is deactuated to decouple the brushless generator from the prime mover and a permanent magnet generator of the brushless generator is provided power at increasing voltage and frequency to accelerate the rotor of the generator. Once a particular speed is reached, power is applied to the exciter and main generator of the brushless generator so that the brushless generator is operated as a motor. Thereafter, the torque converter is actuated so that the motive power developed by the brushless generator is transferred to the prime mover to start same.
Messenger U.S. Pat. No. 3,908,161, discloses a generating system wherein a three-phase alternating voltage is applied to a set of exciter field windings which are connected in a wye configuration. The exciter operates as a rotating transformer, with the armature current of the exciter being rectified and applied to the field winding of the main generator to cause the main generator to operate as a motor and thereby start a prime mover. Once the prime mover has been started, the rotating transformer is converted back to a brushless exciter by changing the connection of the exciter field windings and applying a DC voltage thereto from the output of a permanent magnet generator, or PMG.
Other patents which disclose systems similar to the Messenger device include Lafuze U.S. Pat. Nos. 3,902,073, 3,908,130 and 3,937,974.
A further device which eliminates the necessity for a starter motor is disclosed in Glennon U.S. Pat. No. 4,330,743, assigned to the assignee of the instant application. A reversible AC to DC converter receives external DC power and supplies AC power to drive an alternator as a motor to start a prime mover connected thereto.
Except for the systems disclosed in the Cordner and Aleem patents, in each of the foregoing systems, there is no use made of a constant speed drive connected between the prime mover and the generator. Therefore, these systems do not encounter the problems and inefficiencies associated with starting a prime mover using a generator which is coupled thereto by a constant speed drive.
Hucker et al. U.S. Pat. No. 4,467,267, also assigned to the assignee of the instant application, discloses an alternator excitation system which includes a brushless generator coupled to a prime mover by a constant speed drive. The brushless generator includes separate or integral DC and AC exciters which develop excitation currents which are summed and applied to a field winding of a main generator. The DC exciter includes a field winding which receives DC power provided by a PMG, a rectifier and a voltage regulator while the AC exciter includes a set of field windings which are connected in series with armature windings of the main generator. There is no disclosure, however, that the brushless generator may be operated as a motor to start the prime mover.
Cook U.S. patent application Ser. No. 886,874, filed July 18, 1986, entitled "Inverter Operated Turbine Engine Starting System" and assigned to the assignee of the instant application suggests that a generator coupled to an aircraft engine by a constant speed drive may be used to start the engine provided that additional mechanical components are used to permit the constant speed drive to be operated in reverse or bypassed entirely. These additional components, however, are said to add to the size and weight of the aircraft and may increase aerodynamic drag. Further, no structure is disclosed or suggested to accomplish either result.